Many adaptive braking systems have been proposed for and utilized on vehicles. Some adaptive braking systems include one or more proportioning valves which function in conjunction with the adaptive braking system. Some adaptive braking systems eliminate the need for a proportioning valve. However, should the adaptive braking system fail for any number or variety of reasons, it is highly desirable to have a proportioning valve operating within the braking system so that the rear wheels of the vehicle are prevented from premature lockup during braking of the vehicle, and in the manner in which proportioning valves have been used for many years on vehicles. The present invention provides a solution to the above problem by providing a proportioning valve which is normally inoperative during normal braking and anti-skid braking. However, should the adaptive braking system experience a failure, the proportioning valve is activated and proportions fluid pressure communicated to the rear wheels of the vehicle.